Field of the Invention
This invention relates generally to convective automated and semi-automatic rework systems for the installation and removal of electronic components with respect to a circuit board. More specifically, these components are of the leadless Area Array (or Land Grid Array) type whose means of electronic interconnection with the circuit board is by way of solder balls, solder columns or terminations on the underside of the component body.
Description of Related Art
Representative examples of such component packages include BGAs, PBGAs, CSPs or μBGAs, CBGAs and QFNs. These components are either attached to or removed from the circuit board by heating all the solder interconnections (“solder joints”) simultaneously with a heated fluid (typically air or nitrogen) to some point above solder melt temperature followed by a period of cool down to allow the solder joints to solidify during an installation process, or by separating or lifting the component from the circuit board immediately, well prior to solder solidification during a removal process.
The heated fluid for the solder reflow process is typically provided via an air blower (at a rate of approximately 5-35 SLPM) which passes air through a heater with resistance coil heating elements, where it is heated to a temperature well above solder melt temperature (e.g., 183° C. for Sn63/Pb37 alloy and 217° C. for Sn96.5/Ag3.0/Cu0.5 alloy) and subsequently passed over the component, component mounting site and circuit board to effect solder reflow at the solder joints. In some cases, particularly with high thermal mass circuit boards or components, additional heat is required to be applied to the underside of the circuit board for pre-heating purposes to facilitate and/or hasten the component installation or removal process. Such underside pre-heating is provided by a secondary source of heated fluid, or an IR radiant heating system.
Induction heating methods are well known in various applications such as cooktops, household heating systems, welding and even soldering systems where a workpiece to be soldered (e.g., a steel, copper, brass or aluminum parts) is introduced within an induction coil and heated.
However, induction heating has never before been used to heat a fluid for convective soldering and rework, particularly in a benchtop system specifically for convective rework and with all its attendant advantages over resistance coil heaters discussed below.
The resistance coil heating elements typically employed in such convective reflow systems are often expensive to construct, require costly replacement of the entire element when they fail, have a high incidence of failure, are relatively inefficient at transferring heat to the fluid passing through them and require a great deal of power to operate making them relatively energy inefficient. Resistance coil heating elements are also difficult to control from a temperature standpoint inasmuch as they require relatively robust, high thermal mass construction and thus cannot cool down quickly from a higher temperature setting when a lower temperature setting is subsequently desired.
Due to their relative inefficiency at transferring heat to the fluid (as well as the relative inefficiently of the fluid to transfer heat to the workpiece), such heaters must be overly powered which in turn requires them to be relatively dimensionally large, physically and thermally robust (resistance heating coils must be mounted to a highly heat resistant core such as ceramic or aluminum oxide) and very well insulated or isolated from the rest of the internal components of the rework system so they may withstand significant errant heating, particularly during heavy, continuous use. This also causes the physical size of the reflow head (of the rework system) to grow to disproportionate dimensions (for adequate thermal and electrical isolation) and/or subjects other delicate systems (which by necessity must be in close proximity to the heating element) to excessive heat, degradation and premature failure. A further key negative consequence of such resistance coil heating elements is their very high thermal mass, which causes the heater to take an inordinately long time to heat up from ambient temperature thus delaying throughput of the component rework process. What's more, the high thermal mass acts as a dampener or buffer to the ability of the reworks system to precisely and rapidly control the temperature of the heated fluid and thus better control the component reflow process (during component installation or removal).
This consequence, along with the relatively inefficiency of such heaters to transfer heat to the fluid (as well as the relatively inefficiency of the fluid to transfer heat to the workpiece) as mentioned above, presents a significant challenge to achieving a high level of process control during the rework process which is essential as process requirements varies significantly across various types of circuit boards, components and electronic assemblies.